The present invention relates to a method of efficient bleaching with ozone in an environment friendly bleaching plant comprising oxalic acid addition.
The development of bleaching technique in order to provide more environment friendly processes has in principle followed two different lines of development, one towards the use of chlorine free chemicals, and one towards increased closing of the bleaching plants. Both lines of development have resulted in TCF sequences and so called light ECF sequences. In both of these sequences ozone bleaching is involved as an alternative. One problem with ozone bleaching, however, is that the selectivity evaluated as the pulp viscosity (cellulose degradation) in relation to the lignin release is generally quite poor. Therefore, even if the strength of the pulp per se is not changed to the same extent, this has led to comparatively low interest in ozone bleaching.
In TC and light ECF sequences, a peroxide step is usually included. This step demands the removal of certain metal ions, especially manganese ions. Efficient metal removing steps are either acid steps or complex forming steps, which are also active at low pH values (however, at a higher pH value compared to a pure acid step).
In bleaching, oxalic acid is formed, especially in oxygen delignification, in ozone steps, and in bleaching with chlorine dioxide. The combination of acid steps in which metals are washed out, and there is a high oxalic acid production has led to problems when closing the process, including, for example, calcium oxalate being deposited on the equipment. Calcium oxalate will precipitate at about a pH of from 4-to 8, a pH range in which, on the one hand, the complex forming effect is most efficient, while on the other hand, one which is easily obtained when mixing acid and alkaline bleaching filtrates.
In an article in J. PULP and PAPER SCI. 23(1997); 5 of J. Lidxc3xa9n, a mechanism is explained for the positive effect of Mg addition in oxygen and peroxide bleaching. Magnesium forms a complex with dissolved organic compounds, which in turn can dissolve Mn(II) and Fe(II). Thus, these divalent ions should be dissolved before they can be oxidized to higher valence states in the bleaching steps. The compounds referred to by Lidxc3xa9n et al are formed at alkaline pH.
In laboratory experiments the applicant has made comparisons between additions of different acids to the pulp for obtaining pH value of 3 before ozone bleaching, and the best selectivity was obtained with oxalic acid addition. A subsequent analysis of the different pulps disclosed that the pulp treated with oxalic acid maintained almost the entire original amount of both calcium and manganese ions, despite the low pH value and the fact that the selectivity was the best in this case.
Without being bound to any specific theory it is believed that the oxalic acid added to this step forms complexes with metal ions so that they cannot be washed out and also bind the deleterious divalent metal ions contributing to the decomposition of the ozone.
One object of the present invention is to thus improve the efficiency as compared to prior art, while at the same time the effluents from the bleaching process are diminished or at least are not increased.
In accordance with the present invention, this and other objects have now been realized by the invention of a process for bleaching pulp comprising removing water from the pulp thereby providing a treated pulp and a first filtrate, bleaching the treated pulp with ozone thereby providing a bleached pulp and oxalic acid, washing the bleached pulp with a washing liquid thereby providing a washed bleached pulp and a second filtrate including the oxalic acid, recycling the second filtrate to the pulp prior to the bleaching of the treated pulp, and transferring the first filtrate to the bleached pulp. Preferably, the recycling of the second filtrate comprises recycling the entire amount of the second filtrate.
In accordance with one embodiment of the process of the present invention, the removing of the water from the pulp comprises washing the pulp with a first washing liquid whereby the first filtrate comprises a first washing filtrate and the washing of the bleached pulp comprises washing the bleached pulp with a second washing liquid whereby the second filtrate comprises a second washing filtrate. In a preferred embodiment, the recycling of the second washing filtrate comprises diluting the pulp with the second washing filtrate. In another embodiment, the recycling of the second washing filtrate comprises utilizing at least a portion of the second washing filtrate as the first washing liquid.
In accordance with another embodiment of the process of the present invention, the transferring of the first filtrate to the bleached pulp comprises diluting the bleached pulp with the first filtrate.
In accordance with another embodiment of the process of the present invention, the transferring of the first filtrate to the bleached pulp comprises utilizing at least a portion of the first filtrate as washing liquid for the bleached pulp.
In accordance with another embodiment of the process of the present invention, the process includes adjusting the pH of the treated pulp to a pH of about 3. Preferably, the adjusting of the pH comprises adding a strong acid along with the second filtrate, preferably sulfuric acid.
According to the present invention, these objects are realized by a process in which bleaching filtrate, which is washed out of the pulp after the ozone step, is completely returned to the pulp before bleaching, the filtrate from the washing step before the ozone step is used as washing liquid after the ozone step, or alternatively, in the case where only dewatering is performed before the ozone step, the filtrate from the washing step after the ozone step is used as dilution liquid before the ozone step, for the formation of a closed loop.
In order to provide the correct pH level for ozone bleaching, i.e. about pH 3, a strong acid is also added, preferably sulphuric acid, together with said washing filtrate.